1. Field of the Invention
The present invention relates to an electric power steering system.
2. Description of Related Art
In general, in known electric power steering systems that transmit a motor torque via a worm speed reducer, a gear portion of a resin material integrated with an outer periphery of a cored bar portion of an iron material is used as a worm wheel.
In recent years, the size of the worm wheel has tended to increase with an increase in mechanical output power of the electric power steering system. Consequently, the size and thus the weight of the cored bar portion increase. This may increase an inertia force exerted at the time of steering to degrade a steering feeling.
Thus, an electric power steering system has been developed, which uses aluminum or the like with a lower specific gravity than iron, as the cored bar portion of the worm wheel. In a technique described in Japanese Patent Application Publication No. 2002-2509 (JP 2002-2509 A), an elastic force applying member for a torque limiter is interposed between an iron shaft and an aluminum cored bar.
Japanese Patent Application Publication No. 2001-271903 (JP 2001-271903 A) describes a power transmission system in which the worm wheel includes two components, that is, a gear portion spline-engaged with an auger shaft and a boss portion. Both the boss portion and the gear portion are formed of an iron material. The boss portion is pressed into the gear portion, and both the boss portion and the gear portion rotate relative to each other to provide a torque limiter function when an overtorque load is applied.
If an outer periphery defined by a cylindrical surface of the iron shaft and an inner periphery defined by a cylindrical surface of the cored bar formed of a material softer than iron, for example, aluminum, are entirely press-fitted together along a circumferential direction, the press-fitted surfaces may seize, leading to inappropriate assembly.
This is considered to be because, at the time of press-in, a plastic flow of a surface layer of the soft material member causes a fresh surface to be exposed, resulting in metallic contact between the hard material and the soft material. In particular, when the hard material and the soft material are entirely press-fitted together along the circumferential direction, plastically deformed portions resulting from the plastic flow have nowhere to escape. The plastically deformed portions cut into the press-fitted surface, resulting in a high likelihood of metallic contact and thus seizure.